Gaseous dielectric mixtures for suppressing carbon formation

ABSTRACT

Carbon formation on voltage breakdown and sparking, and consequent carbon deposits on insulators and other surfaces, is suppressed in dielectric gases of halogenated alkanes by adding SF 6  and/or CO 2  to the halogenated alkane to form a gaseous dielectric mixture. Moreover, certain of the gaseous dielectric mixtures evidence unexpectedly high dielectric breakdown voltages. The gaseous dielectric mixtures are useful in high voltage coaxial lines, in transformers, in minisubstations, and the like.

This is a continuation-in-part of application Ser. No. 589,496, filed June 23, 1975, and now abandoned.

BACKGROUND OF THE INVENTION

I. Field of the Invention

This invention relates to a process for the production of dielectric mixtures, useful for preventing or diminishing the formation of carbon in dielectric fluids during electrical discharges therein.

II. Description of the Prior Art

During the operation of electrical equipment, such as switches, circuit breakers, transformers, and the like, arcing, sparking or glow discharges usually or occasionally occur, especially at higher voltages. Dielectric materials are commonly employed to reduce or prevent the possibility of such arcing, sparking and glow discharges. For example, solid insulators, such as ceramics or resins, may be used to support or surround electrical conductors. Or, fluid dielectric materials, such as oils or gases, may be used to insulate electrical conductors.

A related problem involves the breakdown of carbon-containing dielectric materials. During arcing, these materials tend to decompose and form carbon, a non-volatile solid, which, being an electrical conductor, not only shortens the gap between conductors, but also eventually leads to carbon bridge short circuits, or deposited carbon tracks. This is a serious problem which has plagued the electrical industry for years.

As used herein, arc interruption includes arc suppression and arc quenching, and refers to preventing or reducing arcing between electrodes. Carbon formation suppression refers to preventing the formation of carbon during arcing. Suppression of carbon formation also prevents formation of conducting carbon tracks or deposits of non-volatile carbon on insulating surfaces. Such deposits are known to produce regions of non-uniform electric fields which result in a decrease in the dielectric strength of the system.

Sulfur hexafluoride (SF₆) is well-known as an excellent gaseous dielectric. See, e.g., U.S. Pat. No. 3,059,044, issued to R. E. Friedrich et al., Oct. 16, 1962. It is unique in its electric arc interrupting properties. However, SF₆ does have a few inherent limitations: low vapor pressure at low temperatures, comparatively high freezing point (-50.6° C) and relatively high cost.

For some years, it has been known that certain electronegatively substituted carbon compounds (halogenated alkanes) are also highly useful fluid insulators in electrical apparatus. Typical examples are dichlorodifluoromethane (CCl₂ F₂), octafluorocyclobutane (c-C₄ F₈), hexafluoroethane (C₂ F₆), octafluoropropane (C₃ F₈), decafluorobutane (C₄ F₁₀), trichlorofluoromethane (CCl₃ F), sym-dichlorotetrafluoroethane (CClF₂ CClF₂), chloropentafluoroethane (CClF₂ CF₃) and chlorotrifluoromethane (CClF₃). While all of the above have reasonably good dielectric strengths, it is difficult to prevent spark-over or other electrical discharge from occurring in apparatus containing these materials when high voltage surges develop. The spark-over or other discharge typically leads to carbon formation.

A patent issued to J. A. Manion, et al., U.S. Pat. No. 3,650,955, issued Dec. 9, 1966, teaches the use of CCl₂ F₂ combined with c-C₄ F₈ as an arc interrupter gas. However, this combination has been observed to evidence extensive carbon formation properties.

Mixtures of SF₆ and CO₂ have been suggested as a potential gaseous dielectric medium. See, e.g., U.S. Pat. No. 3,059,044, issued to R. E. Friedrich et al., Oct. 16, 1962. However, the patent fails to disclose specific proportions of the components.

Mixtures of insulating gases have been previously disclosed; see, e.g., U.S. Pat. No. 2,173,717, issued Sept. 19, 1939, which discloses mixtures of a gas such as nitrogen or carbon dioxide with other materials such as CCl₂ F₂, and U.S. Pat. No. 3,281,521, issued Oct. 25, 1966, which discloses mixtures of nitrogen, CCl₂ F₂ and SF₆. However, there is no disclosure or suggestion in these patents as to whether carbon formation, which is well-known to occur when carbon-containing gases are exposed to arcing or corona conditions, can be suppressed.

Perhalogenated fluids, including SF₆ and perhalogenated alkanes, have been absorbed on molecular sieves (zeolites), which are then incorporated as fillers in organic insulators; see U.S. Pat. No. 3,305,656, issued to J. C. Devins, Feb. 21, 1967. During high voltage operation, voids in the insulation are filled by the perhalogenated fluid, which then serves as an arc interrupter.

Attempts have been made to develop gaseous dielectric compositions as carbon formation suppressants. For example, B. J. Eiseman, U.S. Pat. No. 3,184,533, issued May 18, 1965, teaches the use of an oxygen-containing oxidizing agent, such as SO₂, N₂ O and NO, to suppress carbon tracing of certain electro-negatively substituted carbon compounds, such as saturated polyhalohydrocarbon compounds, saturated perhalohydrocarbon compounds, saturated perfluoroethers and the like. However, none of these oxidizing agents is desirable because of their corrosive nature, toxicity, and/or chemical reactivity.

In general, any attempts to suppress carbon formation in carbon-containing dielectric gases exposed to arcing or corona conditions by use of a diluent gas requires a high percentage of the diluent gas. Since the well-known diluent gases of nitrogen, carbon tetrafluoride and the like usually have a low dielectric strength, then any gaseous dielectric mixtures employing these diluent gases will consequently have a dielectric strength intermediate the dielectric gas and the diluent gas.

There remains in the art a need for an efficient gaseous dielectric composition that evidences superior carbon suppression properties.

SUMMARY OF THE INVENTION

In accordance with the invention, carbon formation in a dielectric fluid during an electrical discharge from an electrical conductor is suppressed by contacting the electrical conductor during operation with a gaseous dielectric mixture consisting essentially of at least one halogenated alkane plus one member selected from the group consisting of SF₆, in an amount of at least 10 mole percent, CO₂, in an amount of at least 15 mole percent, and a combination of SF₆ and CO₂ which, when plotted on a ternary diagram in mole percent of SF₆ -CO₂ -halogenated alkane, lies in regions rich in SF₆ and CO₂ defined by a line having at its extremities the points defined by

1 SF₆ - 15 CO₂ - 84 halogenated alkane

10 SF₆ - 1 CO₂ - 89 halogenated alkane

Use of SF₆ and/or CO₂ in accordance with the invention permits use of a higher concentration of carbon-containing compounds (halogenated alkanes) than heretofore possible, without formation of carbon resulting from exposure of the gaseous dielectric mixture to arcing and corona conditions. The higher concentration of halogenated alkanes in the mixture permits retention of higher dielectric strengths than otherwise possible.

Halogenated alkanes useful in the practice of the invention are those which contain from 1 to 4 carbon atoms and at most one hydrogen atom, with the remaining hydrogen atoms replaced by at least one halogen selected from the group consisting of fluorine, chlorine or bromine. The halogenated alkanes desirably have a vapor pressure of at least about 100 Torr at 20° C. These compounds are gaseous under operating conditions.

The amount of SF₆ and/or CO₂ required to suppress carbon formation is unique to each mixture. In general, however, for a binary mixture and for multicomponent mixtures containing either SF₆ or CO₂, gaseous mixtures containing at least about 10 mole percent of SF₆ or at least about 15 mole percent of CO₂ are required to suppress carbon formation. For multicomponent mixtures (ternary and higher) containing both SF₆ and CO₂, carbon formation is suppressed for compositions lying in regions rich in SF₆ and CO₂ on a ternary diagram defined by a line having at its extremities the points defined by

1 SF₆ - 15 CO₂ - 84 halogenated alkane

10 SF₆ - 1 CO₂ - 89 halogenated alkane

(the numbers are in mole percent).

Certain of these mixtures form novel compositions. Such novel compositions consist essentially of at least one halogenated alkane plus SF₆ and CO₂. The halogenated alkanes contain from 1 to 4 carbon atoms and at most one hydrogen atom, with the remaining hydrogen atoms replaced by at least one halogen selected from the group consisting of fluorine, chlorine and bromine. The halogenated alkanes desirably have a vapor pressure of at least about 100 Torr at 20° C. The amount of SF₆ and CO₂ in the compositions is as given above.

Further in accordance with the invention, improved dielectric breakdown voltages that are unexpectedly high are obtained by employing specific gaseous dielectric mixtures within the scope of this invention in certain critical proportions set forth below.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1, on coordinates of breakdown voltage in kv-rms and concentration in mole percent, is a plot in a binary system A-B of the dielectric strength of various mixtures of components A and B;

FIG. 2, on coordinates of breakdown voltage in kv-rms and concentration in mole percent, is a plot of various binary mixtures with SF₆ ;

FIG. 3, on coordinates of breakdown voltage in kv-rms and concentration in mole percent, is a plot of various binary mixtures with CO₂ ;

FIG. 4, on coordinates of concentration in mole percent, is a ternary plot of the system SF₆ -CO₂ -CCl₂ F₂, showing useful regions of carbon formation suppression and improved dielectric strength;

FIG. 5, on coordinates of concentration in mole percent, is a ternary plot of the system SF₆ -CO₂ -CHClF₂, showing useful regions of carbon formation suppression and improved dielectric strength;

FIG. 6, on coordinates of concentration in mole percent, is a ternary plot of the system SF₆ -CO₂ -CBrF₃, showing useful regions of carbon formation suppression and improved dielectric strength;

FIG. 7, on coordinates of concentration in mole percent, is a ternary plot of the system SF₆ -CO₂ -CClF₂ CClF₂, showing useful regions of carbon formation suppression and improved dielectric strength;

FIG. 8, on coordinates of concentration in mole percent, is a ternary plot of the system SF₆ -CO₂ -CClF₂ CF₃, showing useful regions of carbon formation suppression and improved dielectric strength;

FIG. 9, on coordinates of concentration in mole percent, is a ternary plot of the system SF₆ -CClF₃ -CHF₃, showing useful regions of carbon formation suppression; and

FIG. 10, on coordinates of concentration in mole percent, is a ternary plot of the system CO₂ -CF₃ CF₃ -c-C₄ F₈, showing useful regions of carbon formation suppression and improved dielectric strength.

DETAILED DESCRIPTION OF THE INVENTION

Dielectric carbon-containing gases decompose under arcing or corona conditions to form carbon deposits. Diluent gases are often combined with the dielectric carbon-containing gases to suppress the carbon formation. However, the mixture of diluent gas and dielectric carbon-containing gas has a low dielectric strength, because the diluent gases, usually nitrogen or the very arc-stable carbon tetrafluoride, themselves have low dielectric strengths, and large quantities of the diluent gases are required in order to suppress carbon formation.

In accordance with the invention, carbon formation suppression in a dielectric fluid during an electrical discharge from an electrical conductor is suppressed by contacting the electric conductor during operation with a gaseous dielectric mixture consisting essentially of at least one halogenated alkane plus one member selected from the group consisting of SF₆, in an amount of at least 10 mole percent, CO₂, in an amount of at least 15 mole percent, and a combination of SF₆ and CO₂ which, when plotted on a ternary diagram in mole percent of SF₆ -CO₂ -halogenated alkane, lies in regions rich in SF₆ and CO₂ defined by a line having at its extremities the points defined by

1 SF₆ - 15 CO₂ - 84 halogenated alkane

10 SF₆ - 1 CO₂ - 89 halogenated alkane.

The dielectric mixtures of the invention permit the retention of the high dielectric strengths associated with the halogenated alkanes, since less suppressant gas (SF₆ and/or CO₂) is needed to suppress carbon formation, as compared with prior art diluent gases. Further, the presence of the suppressant gases of SF₆ and CO₂ apparently serves to saturate any free valences of stripped carbon atoms resulting from the decomposition of the carbon-containing gas by supplying either fluorine atoms (from SF₆) or oxygen atoms (from CO₂), thus preventing the formation of carbon-carbon bonds which would otherwise result in formation of non-volatile carbon deposits on solid surfaces.

Halogenated alkanes useful in the practice of the invention are those which contain from 1 to 4 carbon atoms, since compounds with a greater number of carbon atoms tend to possess undesirably low vapor pressures at desired operating temperatures.

The halogenated alkanes contain at most one hydrogen atom, with the remaining hydrogen atoms replaced by at least one halogen selected from the group consisting of fluorine, chlorine and bromine. More than one hydrogen atom per molecule results in excessive carbon formation.

The halogenated alkanes desirably have a vapor pressure of at least about 100 Torr at 20° C. The vapor pressure limitation permits the use of certain halogenated alkanes, such as 1,1,2-trichloro-1,2,2-trifluoroethane (CCl₂ FCClF₂), which are liquid at room temperature but which evidence a sufficiently high vapor pressure to be useful over a limited range of composition. Preferably, the halogenated alkanes have a vapor pressure of at least about 400 Torr at 20° C, and most preferably, are totally gaseous (760 Torr) at room temperature and have a boiling point of less than about 5° C.

Examples of halogenated alkanes useful in the practice of the invention include chlorodifluoromethane (CHClF₂), bromotrifluoromethane (CBrF₃), hexafluoroethane (CF₃ CF₃) and cyclooctafluorobutane (c-C₄ F₈).

Unexpectedly, in many of these systems, improved breakdown voltage characteristics that are unusually high are obtained by employing specific gaseous dielectric mixtures of this invention within certain critical proportions set forth in examples below. Preferably, perhalogenated alkanes find use in applications such as high dielectric strength mixtures. Perhalogenated compounds are totally halogenated and include no hydrogen. Examples includes chlorotrifluoromethane (CClF₃), 1,2-dichloro-1,1,2,2-tetrafluoroethane (CClF₂ CClF₂), and chloropentafluoroethane (CClF₂ CF₃).

All compositions disclosed herein have utility as gaseous dielectric mixtures for carbon formation suppression. As such, they have application in electrical apparatus, especially high voltage power equipment, such as transformers, capacitors, coaxial lines and minisubstations, having a chamber in which electrical arcing occasionally occurs and which includes the gaseous dielectric mixture. Some of the mixtures are particularly useful in certain specific areas, such as for extreme temperature conditions, when high dielectric strength is required, which are indicated in examples set forth below.

I. Binary Compositions

Binary compositions consist essentially of mixtures of two components, A and B, where A is one member selected from the group consisting of halogenated alkanes and B is one member selected from the group consisting of SF₆ and CO₂. Examples of binary systems preferred as carbon formation suppressants include SF₆ -CCl₂ F₂, SF₆ -CHClF₂, SF₆ -CClF₂ CClF₂, SF₆ -CClF₃, SF₆ -CClF₂ CF₃, CO₂ -CCl₂ F₂ and CO₂ -CHClF₂. While each mixture evidences a unique useful range for carbon formation suppression, in general, at least about 10 mole percent of SF₆ or at least about 15 mole percent of CO₂ is required to obtain suppression. Many mixtures may require somewhat more SF₆ or CO₂. Such a determination is easily within the ability of one skilled in the art, however, and in the Examples section below, details are set forth for determining optimum composition ranges and preferred examples are given; see also Table I below.

Gaseous dielectric mixtures which have a low tendency to form carbon when subjected to repeated electrical sparking (breakdown) are desired for use as carbon formation suppression. This objective is attained by the addition of SF₆ or CO₂ diluent to halogenated alkanes in proper quantities.

Table I summarizes the data developed for binary systems which include SF₆ or CO₂. In Table I are the results of tests of various diluent compounds with potential carbon formation suppression capability, i.e., SF₆, CO₂, SO₂, NO and air. The number listed in Table I in mole percent (suppression value), is the minimum quantity of the diluent component which will prevent carbon formation under the conditions of the tests described in Example 2 below. The stable inert gases, CF₄ and N₂, which also appear in Table I, serve as both diluents and blanks. Inspecting Table I, it is evident that by comparing the suppression values of SF₆ and CO₂ (and others) with those of N₂ and CF₄, the effectiveness of carbon suppression gases and the tendency of various gaseous dielectrics to form carbon can be evaluated. In cases where carbon suppression is most effective, inert diluents generally require a minimum of about 50 to 70 mole percent concentration to suppress carbon formation, compared with a minimum of about 10 to 40 mole percent concentration for suppression of carbon by the diluent-suppressants of the invention. The amount of diluent (SF₆ or CO₂) needed for carbon suppression varies, depending upon the particular halogenated alkane.

                  TABLE I                                                          ______________________________________                                         CARBON FORMATION CONDITIONS                                                    BINARY SYSTEMS                                                                 Compound    SF.sub.6                                                                              CO.sub.2                                                                              N.sub.2                                                                              CF.sub.4                                                                           SO.sub.2                                                                            NO   Air                              ______________________________________                                         CClF.sub.3  10     --     --   --   --   --   --                               CBrF.sub.3  10     15     20   10   --   --   --                               CCl.sub.2 F.sub.2                                                                          10     15     50   50   --   --   --                               CCl.sub.3 F 30     --     --   --   --   --   --                               CHClF.sub.2 35     45     75   75   --   --   --                               CHF.sub.3   10     15     50   50   --   --   --                               CF.sub.3 CF.sub.3                                                                          20     35     30   25   --   --   --                               CClF.sub.2 CF.sub.3                                                                        25     25     50   70   --   --   --                               CClF.sub.2 CClF.sub.2                                                                      45     45     70   70   --   --   --                               c-C.sub.4 F.sub.8                                                                          35     55     70   75   20   20   40                               ______________________________________                                    

From Table I, it is apparent that SF₆ and CO₂ are most effective suppressants with CClF₃, CBrF₃, CCl₂ F₂ and CHF₃. Somewhat more suppressant is required for CF₃ CF₃ and CClF₂ CF₃ and even more suppressant is required for CCl₃ F, CHClF₂, c-C₄ F₈ and CClF₂ CClF₂. In general, less diluent is required to suppress carbon formation when SF₆ or CO₂ is employed than when N₂ or CF₄ is employed. With c-C₄ F₈, it is possible to compare the effectiveness of SF₆ and CO₂ with the suppressant gases of SO₂ and NO of U.S. Pat. 3,184,533 and with air. The accuracy of suppression values is ± 5 percent. Thus, SF₆ is only somewhat less effective than NO or SO₂. Carbon dioxide has about the same effect as air.

However, sulfur dioxide (SO₂) is a toxic, corrosive gas and is thus undesirable in a practical system. Nitric oxide (NO), also toxic and corrosive, is chemically unstable. Nitrous oxide (N₂ O), also chemically unstable, is an anesthetic. Air is undesirable since it tends to attack equipment components such as metals and plastics, particularly at the usual operating range of 120° to 250° C.

Without subscribing to any particular theory, it is possible that since SF₆ is an inert diluent up to about 200° C, and CO₂ is an inert diluent up to about 300° C, their action in carbon suppression is probably the formation of fluorine or oxygen atoms under arc conditions. These atoms then subsequently react with the carbon-containing fragments of the arced halogenated alkanes, thereby forming non-conducting decomposition products rather than electrically conducting carbon.

Since halogenated alkanes vary in their carbon formation tendencies, the desired composition ranges are conveniently based upon the carbon suppression values of Table I. That is, for SF₆ mixtures, the broad range of compositions useful as dielectric gases varies from the minimum diluent necessary to suppress carbon formation up to about 99 mole percent of SF₆. For CO₂ mixtures, compositions having a breakdown voltage of greater than about 10 kv-rms (kilovolt-root mean square) are considered useful, except in certain special applications. Generally, compositions containing at least the minimum amount of CO₂ necessary to suppress carbon formation, but less than about 65 to 80 mole percent of CO₂, depending on the particular gaseous mixture, are considered useful.

Of course, operating at voltages considerably less than the breakdown voltage at which carbon formation appears would permit use of a somewhat broader range of compositions. Preferred compositions are those that retain about 90% of the breakdown voltage of the higher of the two components.

Within the broad range disclosed above, many mixtures of halogenated alkanes with SF₆ and with CO₂ evidence an unexpected enhancement of dielectric strength, as measured by breakdown voltage, using a standard cell as described by ASTM D2477-66T. Examples of such systems include SF₆ -CCl₂ F₂, SF₆ -CBrF₃ and and CO₂ -CBrF₃. It would be expected that for most binary compositions, breakdown voltage would vary linearly with composition. However, for some compositions, an unexpected enhancement of breakdown voltage is observed. This may take the form either of a moderate positive deviation from linearity or of a significant positive deviation from linearity to the extent that over some range of composition, the observed breakdown voltage is equal to or greater than that of either of the two end members. The latter condition is referred to herein as a synergistic effect. It is not possible to indicate general composition ranges. However, such a determination for a specific system is easily within the ability of one skilled in th art. The Examples section sets forth further details and lists preferred examples; see also Tables IV and V, below.

An example of both carbon formation suppression and improved dielectric strength in accordance with the invention is shown in FIG. 1, which is a plot of breakdown voltage in kv-rms as a function of composition in mole percent for a binary system of components A and B. Carbon formation appears over the range indicated by the dotted portions of the curves. In this example, component B is chloropentafluoroethane (CClF₂ CF₃). Component A is variously SF₆ (curve 10); CO₂ (curve 11); and CF₄ (curve 12). Where component A is SF₆ (curve 10), there is not only a positive deviation from linearity (cf. line 13), but an actual enhancement such that the mixture over a range of composition evidences a breakdown voltage greater than that of either of the two end members. Where component A is CO₂ (curve 11), there is a positive deviation from linearity. Where component A is CF₄ (curve 12), both extensive carbon formation and little deviation from linearity are observed. Line 13 depicts the expected linear behavior of breakdown voltage with composition variation. Such results for binary mixtures are typical of many of the mixtures of halogenated alkanes with SF₆ and CO₂ disclosed herein. Such mixtures tend to exhibit both low carbon formation and ehnhanced breakdown voltage characteristics.

FIGS. 2 and 3 depict preferred binary systems with SF₆ and CO₂, respectively. In FIG. 2, the following curves represent the breakdown voltages of the listed compositions with SF₆ : curve 20, dichlorodifluoromethane (CCl₂ F₂); curve 21, chlorodifluoromethane (CHClF₂); curve 22, 1,2-dichloro-1,1,2,2-tetrafluoroethane (CClF₂ CClF₂); curve 23, chlorotrifluoromethane (CClF₃); and curve 24, chloropentafluoroethane (CClF₂ CF₃). In FIG. 3, the following curves represent the breakdown voltages of the listed compositions with CO₂ : curve 30, CHClF₂ and curve 31, CCl₂ F₂.

II. Multi-Component Compositions

Ternary compositions consist essentially of mixtures of three compounds, A, B and C, at least one of which is selected from the group consisting of halogenated alkanes and at least one of which is selected from the group consisting of SF₆ and CO₂. Examples of ternary systems preferred as carbon formation suppressants include SF₆ -CO₂ -CCl₂ F₂, SF₆ -CO₂ -CHClF₂, SF₆ -CO₂ -CClF₂ CClF₂, SF₆ -CO₂ -CClF₂ CF₃, SF₆ -CO₂ -CBrF₃, SF₆ -CClF₃ -CHF₃, CO₂ -CCl₂ F₂ -CHClF₂ and CO₂ -CF₃ CF₃ -c-C₄ F₈. While each mixture evidences a unique useful range for carbon formation suppression, in general, for multicomponent mixtures containing either SF₆ or CO₂, gaseous mixtures containing at least about 10 mole percent of SF₆ or at least about 15 mole percent of CO₂ are required to suppress carbon formation. For multicomponent mixtures containing both SF₆ and CO₂, carbon formation is suppressed for compositions lying in regions rich in SF₆ and CO₂ on a ternary diagram defined by a line having at its extremities the points defined by

1 SF₆ - 15 CO₂ - 84 halogenated alkane

10 SF₆ - 1 CO₂ - 89 halogenated alkane.

Many mixtures may require somewhat more SF₆ and/or CO₂. As before, such a determination is within the ability of one skilled in the art. The Examples section below sets forth the details for determining such ranges and lists preferred examples.

Within the broad range of compositions useful for carbon formation suppression, many ternary mixtures evidence an unexpected enhancement of dielectric strength. Preferred examples of these systems include SF₆ -CO₂ -CCl₂ F₂, SF₆ -CO₂ -CHClF₂, SF₆ -CO₂ -CClF₂ CClF₂, SF₆ -CO₂ -CClF₂ CF₃, SF₆ -CO₂ -CBrF₃, SF₆ -CHF₃ -CHClF₂ and CO₂ -CF₃ CF₃ -c-C₄ F₈. Mixture possessing this property are also listed in the Examples section.

An example of both carbon formation suppression and improved dielectric strength in accordance with the invention is shown in FIG. 4, which is a plot of breakdown voltage in kv-rms as a function of composition in mole percent for the ternary system SF₆ -CO₂ -CCl₂ F₂. Carbon formation appears for compositions rich in CCl₂ F₂, defined by a line having at its extremities the points defined by

b. 1 SF₆ - 15 CO₂ - 84 CCCl₂ F₂

c. 1 SF₆ - 1 Co₂ - 89 CCl₂ F₂.

This system evidences useful dielectric behavior within an area on the ternary diagram defined by a polygon a-b-c-d-e-a having at its corners the points defined by:

a. 1 SF₆ - 65 CO₂ - 34 CCl₂ F₂

b. 1 SF₆ - 15 CO₂ - 84 CCl₂ F₂

c. 10 SF₆ - 1 CO₂ - 89 CCl₂ F₂

d. 98 SF₆ - 1 CO₂ - 1 CCl₂ F₂

e. 24 SF₆ - 75 CO₂ - 1 CCl₂ F₂.

There is a synergistic BDV effect within an area on the ternary diagram defined by a polygon f-g-d-h-f having at its corners the points defined by

f. 30 SF₆ - 25 CO₂ - 45 CCl₂ F₂

g. 30 SF₆ - 1 CO₂ - 69 CCl₂ F₂

d. 98 SF₆ - 1 CO₂ - 1 CCl₂ F₂

h. 74 SF₆ - 25 CO₂ - 1 CCl₂ F₂.

See also Examples 2 and 21, below.

Other preferred examples are depicted in FIGS. 5 through 10. The figures are associated with the following systems, which are explained in further detail in the Examples section below: FIG. 5, SF₆ -CO₂ -CHClF₂ (Example 22); FIG. 6, SF₆ -CO₂ -CBrF₃ (Example 23); FIG. 7, SF₆ -CO₂ -CClF₂ CClF₂ (Example 25); FIG. 8, SF₆ -CO₂ -CClF₂ CF₃ (Example 26); FIG. 9, SF₆ -CClF₃ -CHF₃ (Example 27) and FIG. 10, CO₂ -CF₃ CF₃ -c-C₄ F₈ (Example 34).

Quaternary and higher compositions within the above definition may also be formulated in accordance with the invention. One such example is SF₆ -CO₂ -CCl₂ F₂ -CClF₂ CF₃ (Example 36).

The considerations in choosing a particular system include the cost of the components, the temperature performance desired (low or high), the electrical properties desired, and the relative saftey of the total mixture.

EXAMPLES I. Description of Test Procedure

Breakdown voltage (BDV) was measured by equipment which included a glass breakdown voltage cell as described by ASTM D2477-66T, a 50 kv-rms (kilovolt-root mean square), 60 Hz, 5 kva transformer and suitable accessory circuits. A vacuum manifold with Bourdon Tube type manometer, solenoid valves and controls was also used.

The cell had an 0.75 inch sphere and a 1.5 inch plane electrode. The breakdown cell filling manifold, using solenoid valves, furnished connections to the cell, the manometer, various gas inlets and the vacuum pump. The manometer was a Wallace and Tiernan model 62A-4D-0800, ranging in two rotations of the indicator needle between 0 and 800 Torr absolute. A simple control panel governed the solenoid valves used to admit the various gases of the mixtures in the BDV cell. The BDV measurement conditions were 60 Hz, 0.100 inch gap, 760 Torr total pressure and ambient room temperature. Compositions were prepared in terms of partial pressure, accurate to ± 0.5 Torr, and converted to mole percent.

The electrodes had to be polished prior to taking BDV data. They were polished with E5 emergy grit, soaked in xylene for 30 min, rinsed with petroleum ether and dried at 100° C for 15 min. A few preliminary breakdown voltage shots were necessary prior to taking data to condition the electrodes. Even so, the BDV of pure components, such as SF₆, was observed to vary slightly from one experiment to the next.

For measuring carbon formation suppression, there were two levels of testing. In the first, any carbon appearing after 5 BDV shots was monitored as a "go-no go" test. For a more severe exposure test, 50 successive BDV shots were taken in the same manner.

Carbon tetrafluoride, CF₄, the most stable fluorocarbon known, and nitrogen, N₂, served as inert diluents and blanks. In the test for carbon formation, the measurements started at high SF₆ or CO₂ concentrations. These were gradually reduced until carbon deposits appeared. Carbon was usually observed to form on the grounded plane electrode.

EXAMPLE 1 Procedure for Measurement of Breakdown Voltage

This Example demonstrates the breakdown voltage measurement by the ASTM D2477-66T method, using a mixture of SF₆ and CCl₂ F₂. The equipment included a vacuum manifold, the glass breakdown voltage cell, 0 to 50 kv test set rated at 5 kva and 40,000 ohms of 250 watt current limiting resistors. The manifold had valved connections to air, to the vacuum pump, to the manometer and to three cylinders which contained components A, B or C.

An air gap of two 12.5 cm diameter brass spheres served for a peak voltage calibration standard. Prior to measurement, the transformer's voltmeters were calibrated with this gap using the BDV methods of ASTM D2477-66T, i.e., averaging 5 successive spark breakdowns at set gap distances. The voltmeters were accurate to 0.5 kv, or within calibration

In preparing a test sample, the ideal gas law was used, and pressure percent was assumed equal to mole percent. The desired mole percent of each component was calculated as the number of Torr compared to 760 Torr (1 atmosphere), which yielded the desired mole percent. Prior to make-up of the composition, the test cell was evacuated to less than 1 Torr. During make-up of the composition, the component to be present in the smallest amount was admitted first, until it attained the desired partial pressure. This was followed by the component with the next highest percentage and finally by the component present in the largest mole fraction. Table II below presents the pressures used for some SF₆ -CCl₂ F₂ mixtures, together with the breakdown voltage of each composition and its standard deviation (SD).

                  TABLE II                                                         ______________________________________                                         BREAKDOWN VOLTAGE OF SF.sub.6 -CCl.sub.2 F.sub.2 MIXTURES                      SF.sub.6    CCl.sub.2 F.sub.2                                                                            BDV,     ± SD,                                    Mole % P,Torr   Mole %   P,Torr Kv-rms*                                                                               Kv-rms*                                 ______________________________________                                         100    760      0         0     17.43  0.33                                    80     608      20       152    17.74  0.51                                    60     456      40       304    18.53  0.29                                    40     304      60       456    18.46  0.51                                    20     152      80       608    17.90  0.43                                    0      0        100      760    17.28  0.32                                    ______________________________________                                          *rms = root mean square value, i.e. BDV rms = 0.707 BDV peak.            

Synergism is indicated in the magnitude of about 1 kv-rms greater than the breakdown voltage of SF₆ over the range of about 40 to 60 mole percent of SF₆ ; see also FIG. 2 and Example 4, below.

EXAMPLE 2 Process for Measurement of Carbon Formation Suppression

This Example describes the method of measuring carbon formation suppression, using SF₆, CO₂ and CCl₂ F₂. The equipment of Example 1 was used for the tests. The compositions were again made up using pressure percent (mole percent) at one atmosphere total pressure. To evaluate carbon formation, a given sample of definite composition was repeatedly sparked, as in Example 1, and BDV observed. There were two levels of exposure, 10 sparks and 50 sparks, all applied successively to the same gas sample. If carbon appeared, the BDV cell was disassembled and the electrodes cleaned and conditioned again.

Table III presents the pressures and compositions of the samples, the observed breakdown voltages and the number of shots which did, or did not, produce carbon. With these mixtures, a 5 percent change in composition caused a large increase in carbon formation suppression: at 90 CCl₂ F₂ - 10 CO₂, carbon formed after 20 sparks, whereas at 85 CCl₂ F₂ - 15 CO₂, no carbon appeared after 50 sparks. Similarly, pure CF₂ Cl₂ formed carbon after 10 sparks, while at 95 CCl₂ F₂ - 5 SF₆, no carbon appeared after 50 sparks. A detailed study of this system is shown in FIG. 4 and is discussed below in further detail in EXample 21. In FIG. 4, the breakdown voltages of compositions in the system SF₆ -CO₂ -CCl₂ F₂ are depicted on a ternary plot as a function of mole percent.

                                      TABLE III                                    __________________________________________________________________________     CARBON FORMATION RESULTS FOR SF.sub.6 -CO.sub.2 -CCl.sub.2 F.sub.2             MIXTURES                                                                       COMPOSITIONS                  ELECTRICAL                                                                              CRABON                                  SF.sub.6  CCl.sub.2 F.sub.2                                                                        CO.sub.2  BDV, SD, Number of Sparks                        mole %                                                                              P, Torr                                                                             mole %                                                                              P, Torr                                                                             mole %                                                                              P, Torr                                                                             kv   kv  C Forms                                                                             No C                               __________________________________________________________________________     100  760   0   --   0    --   17.43                                                                               0.3 --   --                                 0    --   100  760  0    --   17.43                                                                               0.3 10   --                                 5     38  95   722  0    --   16.89                                                                               0.7 --   50                                 0    --   90   684  10   76   15.04                                                                               1.0 20   --                                 0    --   85   646  15   114  13.28                                                                               0.7 --   50                                 50   380  46     349.6                                                                             4    30.4 18.71                                                                               0.4 --   50                                 75   570  20   152  5    38.0 20.20                                                                               1.0 --   50                                 __________________________________________________________________________

II. Binary Mixtures A. SF₆ Binary Mixtures

The breakdown voltage data for binary mixtures which included SF₆ is listed in Table IV. From the data given, both the minimum amount of SF₆ useful in suppressing carbon formation and the useful range for gaseous dielectric behavior may be determined. Many binary mixtures evidenced breakdown voltage values within about 90% of that of the higher end member over a range of compositions; such mixtures are preferred. Certain binary mixtures evidenced unexpectedly high breakdown voltage values compared with the values of either end member. Since the normal expected behavior is a linear dependence with composition, such unusual behavior is termed a synergistic effect, and such mixtures are also preferred. Following Table IV is a discussion of some of the binary mixtures including SF₆ and their utility.

                                      TABLE IV                                     __________________________________________________________________________     SF.sub.6 BINARY MIXTURES                                                       Breakdown Voltage, kv-rms, as a Function of SF.sub.6 Addition                                                                    Min. Diluent,                Composition                                                                           0   10  20  25  30  40 50 60 70 75 80 90 100                                                                              Mole %                       __________________________________________________________________________     CCl.sub.3 F                                                                           23.66*                                                                             23.77*                                                                             23.45*                                                                             23.32*  23.23                                                                             23.50                                                                             21.98                                                                             21.00                                                                             21.00                                                                             20.80                                                                             18.17                                                                             18.10                                                                             30                          CCl.sub.2 F.sub.2                                                                     14.78*                                                                             15.75                                                                              15.97   16.32                                                                              16.60                                                                             16.40                                                                             16.73                                                                             16.82 16.53                                                                             16.40                                                                             16.10                                                                             10                          CClF.sub.3                                                                            7.14*                                                                              7.66                                                                               11.95   13.86                                                                              14.63                                                                             15.04                                                                             14.70                                                                             15.32 15.24                                                                             15.65                                                                             15.79                                                                             10                          CBrF.sub.3                                                                            13.50*                                                                             13.78                                                                              15.21   15.45                                                                              14.70                                                                             15.34                                                                             18.26                                                                             18.96 19.43                                                                             16.80                                                                             16.79                                                                             10                          CHClF.sub.2                                                                           4.79*                                                                              8.88*                                                                              10.70*  13.11*                                                                             15.41                                                                             16.58                                                                             16.36                                                                             16.51 16.58                                                                             16.36                                                                             16.32                                                                             35                          CHF.sub.3                                                                             5.74*                                                                              6.65                                                                               7.51    10.70                                                                              12.56                                                                             13.56                                                                             14.40                                                                             14.91 14.76                                                                             15.39                                                                             16.32                                                                             10                          CCl.sub.2 FCClF.sub.2         20.53                                                                             19.94                                                                             18.76 17.66                                                                             16.99                                                                             16.32                                                                             50                          CClF.sub.2 CClF.sub.2                                                                 21.84*                                                                             21.55*                                                                             21.41*  21.23*                                                                             20.94                                                                             20.62                                                                             20.40                                                                             20.15 19.86                                                                             19.07                                                                             16.61                                                                             45                          CClF.sub.2 CF.sub.3                                                                   16.72*                                                                             16.83*                                                                             17.34*  17.92                                                                              20.51                                                                             20.06                                                                             18.08                                                                             16.99 17.25                                                                             17.41                                                                             16.90                                                                             25                          CF.sub.3 CF.sub.3                                                                     15.0*       15.8       16.3     17.0     16.9                                                                              20                          c-C.sub.4 F.sub.8                                                                     19.90*                                                                             18.53*                                                                             18.60*  19.54*                                                                             19.92                                                                             19.97                                                                             19.50                                                                             18.92 18.78                                                                             17.92                                                                             17.70                                                                             35                          CF.sub.4                                                                              10.8        11.3       14.7     17.5     19.1                                                                              --                          N.sub.2                                                                               8.5         13.7       18.9     20.4     21.9                                                                              --                          CO.sub.2                                                                              6.20                                                                               8.50                                                                               10.19   11.39                                                                              12.46                                                                             14.20                                                                             16.38                                                                             16.81 16.83                                                                             16.87                                                                             16.56                                                                             --                          __________________________________________________________________________      *Carbon formation observed.                                              

EXAMPLE 3 System SF₂ -CCl₃ F

A BDV of 23.7 kv was observed for CCl₃ F, compared with a value of 18.1 kv for SF₆. The system evidenced useful dielectric behavior over the range of about 30 to 99 mole percent of SF₆. The BDV was at least 90% that of CCl₃ F over the range of about 30 to 70 mole percent of SF₆. At least about 30 mole percent of SF₆ was required to suppress carbon formation in CCl₃ F.

The combination of SF₆ and CCl₃ F is an inexpensive dielectric mixture. The preferred operating temperature range is greater than ambient but less than 150° C.

EXAMPLE 4 System SF₆ -CCl₂ F₂ (FIG. 2, curve 20)

This system evidenced useful dielectric behavior over the range of about 10 to 99 mole percent of SF₆. There was a synergistic BDV effect over the range of about 40 to 80 mole percent of SF₆. At least about 10 mole percent SF₆ was required to suppress carbon formation in CCl₂ F₂.

The combination of SF₆ and CCl₂ F₂ is an inexpensive dielectric mixture for use in units such as underground or underwater high voltage coaxial lines, capacitors and in gas filled transformers.

EXAMPLE 5 System SF₆ -CClF₃ (FIG. 2, curve 23)

This system evidenced useful dielectric behavior over the range of about 15 to 99 mole percent of SF₆. The BDV was at least 90% that of SF₆ over the range of about 60 to 99 mole percent of SF₆. There was a synergistic BDV effect over a narrow range of about 75 to 85 mole percent of SF₆. At least about 15 mole percent of SF₆ was required to suppress carbon formation in CClF₃.

This system is useful in raising the vapor pressure of SF₆ without substantially decreasing the BDV. Hence, it is suitable for low temperature use in transformers and capacitors.

EXAMPLE 6 System SF₆ -CBrF₃

This system evidenced useful dielectric behavior over the range of about 10 to 99 mole percent of SF₆. The BDV was at least 90% that of SF₆ over the range of about 20 to 99 mole percent of SF₆. There was a synergistic BDV effect over the range of about 60 to 85 mole percent of SF₆. At least about 10 mole percent of SF₆ was required to suppress carbon formation in CBrF₃.

EXAMPLE 7 System SF₆ -CHClF₂ (FIG. 2, curve 21)

An SF₆ -CHClF₂ azeotrope existed at 90 mole percent of SF₆. This system evidenced useful dielectric behavior over the range of about 35 to 99 mole percent SF₆. A synergistic effect was observed over a narrow range of about 40 to 50 mole percent of SF₆. At least about 35 mole percent of SF₆ was required to suppress carbon formation in CHClF₂.

The combination of SF₆ and CHClF₂ is an inexpensive dielectric mixture with only a slight compromise in SF₆ BDV and vapor pressure.

EXAMPLE 8 System SF₆ -CHF₃

This system evidenced useful dielectric behavior over the range of about 15 to 99 mole percent of SF₆. The BDV was at least 90% that of SF₆ over the range of about 65 to 99 mole percent of SF₆. At least about 15 mole percent of SF₆ was required to suppress carbon formation in CHF₃.

The use of CHF₃ can increase the vapor pressure of SF₆ without substantial SF₆ BDV decrease, either alone or together with an azeotropic mixture of CClF₃. These gaseous dielectric mixtures are useful in low temperature applications, such as gas filled transformers which are exposed to winter conditions.

EXAMPLE 9 System SF₆ -CClF₂ CClF₂ (FIG. 2, curve 22)

A BDV of 21.8 kv was observed for CClF₂ CClF₂, compared with a value of 16.6 kv for SF₆. The system evidenced useful dielectric behavior over the range of about 45 to 99 mole percent of SF₆. The BDV was at least 90% that of CClF₂ CClF₂ over the range of about 45 to 85 mole percent of SF₆. At least about 45 mole percent of SF₆ was required to suppress carbon formation in CClF₂ CClF₂. This system evidenced a substantial BDV improvement over SF₆ alone.

The relatively high boiling point of CClF₂ CClF₂ (3.6° C) limits low temperature uses of this system, but at ambient room temperature or above, it is a satisfactory dielectric mixture at only moderate cost.

EXAMPLE 10 System SF₆ -CClF₂ CF₃ (FIGS. 1, curve 10, and 2, curve 24)

This system evidenced useful dielectric behavior over the range of about 25 to 99 mole percent of SF₆. There was a synergistic effect over the range of about 25 to 90 mole percent of SF₆, and a substantial synergistic effect (greater than about 1 kv) over the range of about 30 to 60 mole percent of SF₆. At least about 25 mole percent of SF₆ was required to suppress carbon formation in CClF₂ CF₃.

The combination of SF₆ and CClF₂ CF₃ is an inexpensive gaseous dielectric mixture having the same or higher dielectric strength than SF₆ alone.

EXAMPLE 11 System SF₆ -CF₃ CF₃

This system evidenced useful dielectric behavior over the range of about 20 to 99 mole percent of SF₆. At least about 20 mole percent SF₆ was required to suppress carbon formation in CF₃ CF₃.

Due to the low boiling point of CF₃ CF₃ (-78° C) and its good thermal stability, mixtures of CF₃ CF₃ with SF₆ are desirable for low temperature applications. Also, since CF₃ CF₃ is a very thermally stable gas, the addition of sufficient SF₆ to suppress carbon formation (about 20 mole percent) should lead to a more desirable high temperature gaseous dielectric mixture for use in transformers.

EXAMPLE 12 System SF₆ -c-C₄ F₈

A BDV of 19.9 kv was observed for c-C₄ F₈, compared with a value of 17.7 kv for SF₆. The system evidenced useful dielectric behavior over the range of about 35 to 99 mole percent of SF₆. At least about 35 mole percent of SF₆ was required for carbon formation suppression.

The SF₆ -c-C₄ F₈ system has a higher BDV than SF₆ alone. It is not suitable for use below 0° C due to the high boiling point of c-C₄ F₈ (-60° C). On the other hand, c-C₄ F₈ can be a component in high temperature gaseous dielectric mixtures; see also its use with CO₂ in Example 20, below.

B. CO₂ Binary Mixtures

The breakdown voltage data for binary mixtures which included CO₂ are listed in Table V. From the data given, both the minimum amount of CO₂ useful in suppressing carbon formation and the useful range for gaseous dielectric behavior may be determined. As before, mixtures evidencing at least 90% of the breakdown voltage of the higher of the two components are preferred, as are synergistic compositions. Following Table V is a discussion of some of the binary mixtures including CO₂ and their utility.

In general, while CO₂ binary mixtures tended to evidence less BDV synergism than did the SF₆ binary mixtures, they evidenced good carbon formation suppression properties. Except in special applications, such as low voltage use, mixtures evidencing breakdown voltages of less than about 10 kv-rms are not considered to be as useful as those greater than about 10 kv-rms.

                                      TABLE V                                      __________________________________________________________________________     CO.sub.2 BINARY MIXTURES                                                       Breakdown Voltage, kv-rms, as a Function of CO.sub.2 Addition                                                                 Min. Diluent,                   Composition                                                                           0   10  20  30  40  50  60  70 80 90 100                                                                               Mole %                          __________________________________________________________________________     CCl.sub.2 F.sub.2                                                                     14.78*                                                                             14.89*                                                                             14.91                                                                              13.65                                                                              12.22                                                                              11.17                                                                              10.73                                                                              9.54                                                                              8.92                                                                              8.06                                                                              5.74                                                                              15                              CBrF.sub.3                                                                            13.50*                                                                             11.85*                                                                             11.19                                                                              11.00                                                                              10.79                                                                              10.38                                                                              10.59                                                                              9.52                                                                              7.43                                                                              6.72                                                                              5.92                                                                              15                              CHClF.sub.2                                                                           5.50*                                                                              6.00*                                                                              7.06*                                                                              8.02*                                                                              10.85*                                                                             11.02                                                                              10.21                                                                              9.96                                                                              8.60                                                                              7.04                                                                              6.20                                                                              45                              CHF.sub.3                                                                             5.90*                                                                              6.60*                                                                              6.72                                                                               6.10                                                                               5.68                                                                               5.70                                                                               5.76                                                                               5.80                                                                              5.78                                                                              5.76                                                                              5.68                                                                              15                              CCl.sub.2 FCClF.sub.2      13.58*                                                                             12.05*                                                                             11.10                                                                             9.72                                                                              8.22                                                                              6.16                                                                              65                              CClF.sub.2 CClF.sub.2                                                                 21.84*                                                                             20.87*                                                                             19.83*                                                                             18.71*                                                                             17.50*                                                                             15.54                                                                              14.40                                                                              13.33                                                                             11.08                                                                             9.04                                                                              6.22                                                                              45                              CClF.sub.2 CF.sub.3                                                                   16.72*                                                                             16.10*                                                                             16.03*                                                                             15.39                                                                              14.28                                                                              12.78                                                                              11.17                                                                              10.00                                                                             9.20                                                                              8.08                                                                              6.07                                                                              25                              CF.sub.3 CF.sub.3                                                                     16.81*                                                                             15.90*                                                                             14.16*                                                                             12.07*                                                                             10.85                                                                              10.04                                                                              8.91                                                                               8.44                                                                              7.68                                                                              6.82                                                                              5.70                                                                              35                              c-C.sub.4 F.sub.8                                                                     16.58*                                                                             16.83*                                                                             17.74*                                                                             16.63*                                                                             16.27*                                                                             18.82*                                                                             17.09                                                                              13.11                                                                             9.37                                                                              9.00                                                                              5.70                                                                              55                              __________________________________________________________________________      *Carbon formation observed                                               

EXAMPLE 13 System CO₂ -CCl₂ F₂ (FIG. 3, curve 31)

This system evidenced useful dielectric behavior over the range of about 15 to 65 mole percent of CO₂. The BDV was at least 90% that of CCl₂ F₂ over the range of about 15 to 35 mole percent of CO₂. At least about 15 mole percent of CO₂ was required to suppress carbon formation in CCl₂ F₂.

At about 20 mole percent of CO₂, this system has a BDV of 14.9 kv, compared with a BDV of 16.6 kv for pure SF₆. This system is an inexpensive gaseous dielectric mixture suitable for operation in the range of about -20° to 150° C.

EXAMPLE 14 System CO₂ -CBrF₃

This system evidenced useful dielectric behavior over the range of about 15 to 65 mole percent of CO₂. At least about 15 mole percent of CO₂ was required to suppress carbon formation in CBrF₃.

This binary system is useful in low temperature applications.

EXAMPLE 15 System CO₂ -CHClF₂ (FIG. 3, curve 30)

This system evidenced useful dielectric behavior over the range of about 45 to 70 mole percent of CO₂. There was a synergistic effect over this entire range. At least about 45 mole percent of CO₂ was required to suppress carbon formation in CHClF₂.

This system is an inexpensive dielectric mixture for low voltage uses when SF₆ is not economically practical.

EXAMPLE 16 System CO₂ -CHF₃

This system evidenced useful dielectric behavior over the range of about 15 to 99 mole percent of CO₂. There was a synergistic effect over a narrow range of about 15 to 25 mole percent of CO₂. At least about 15 mole percent of CO₂ was required to suppress carbon formation in CHF₃. PG,31

EXAMPLE 17 System CO₂ -CClF₂ CClF₂

This system evidenced useful dielectric behavior over the range of about 45 to 85 mole percent of CO₂. At least about 45 mole percent of CO₂ was required to suppress carbon formation in CClF₂ CClF₂.

EXAMPLE 18 System CO₂ -CClF₂ CF₃ (FIG. 1, curve 11)

This system evidenced useful dielectric behavior over the range of about 25 to 70 mole perecent of CO₂. The BDV was at least 90% that of CClF₂ CF₃ over the range of about 25 to 35 mole percent of CO₂. At least about 25 mole percent of CO₂ was required to suppress carbon formation in CClF₂ CF₃.

This system is suitable for dry type transformers up to about 250° C and is relatively inexpensive compared with SF₆.

EXAMPLE 19 System CO₂ -CF₃ CF₃

This system evidenced useful dielectric behavior over the range of about 35 to 50 mole percent of CO₂. At least about 35 mole percent of CO₂ was required to suppress carbon formation in CF₃ CF₃.

EXAMPLE 20 System CO₂ -c-C₄ F₈

This system evidenced useful dielectric behavior over the range of about 55 to 75 mole percent of CO₂. At least about 55 mole percent CO₂ was required to suppress carbon formation in c-C₄ F₈.

This system can be used in formulating multi-component mixtures which do not contain SF₆ and which are suitable for operating temperatures up to about 300° C.

III. Multicomponent Mixtures

Data for these mixtures are most conveniently represented on ternary diagrams expressed in mole percent.

EXAMPLE 21 System SF₆ -CO₂ -CCl₂ F₂ (FIG. 4)

This system evidenced useful dielectric behavior within an area on a ternary diagram defined by a polygon a-b-c-d-e-a having at its corners the points defined by

a. 1 SF₆ - 65 CO₂ - 34 CCl₂ F₂

b. 1 SF₆ - 15 CO₂ - 84 CCl₂ F₂

c. 10 SF₆ - 1 CO₂ - 89 CCl₂ F₂

d. 98 SF₆ - 1 CO₂ - 1 CCl₂ F₂

e. 24 SF₆ - 75 CO₂ - 1 CCl₂ F₂.

There was a synergistic BDV effect within an area on the ternary diagram defined by a polygon f-g-d-h-f having at its corners the points defined by

f. 30 SF₆ - 25 CO₂ - 45 CCl₂ F₂

g. 30 SF₆ - 1 CO₂ - 69 CCl₂ F₂

d. 98 SF₆ - 1 CO₂ - 1 CCl₂ F₂

h. 74 SF₆ - 25 CO₂ - 1 CCl₂ F₂.

Carbon formation was suppressed for compositions lying in regions rich in SF₆ and CO₂ defined by a line b-c having at its extremities the points defined by

b. 1 SF₆ - 15 CO₂ - 84 CCl₂ F₂

c. 10 SF₆ - 1 CO₂ - 89 CCl₂ F₂.

This system is an inexpensive gaseous dielectric mixture suitable for coaxial lines exposed to temperatures down to -30° C.

EXAMPLE 22 System SF₆ CO₂ -CHClF₂ (FIG. 5)

This system evidenced useful dielectric behavior within an area on a ternary diagram defined by a polygon a-b-c-d-e-a having at its corners the points defined by

a. 1 SF₆ - 70 CO₂ - 29 CHClF₂

b. 1 SF₆ - 50 CO₂ - 49 CHClF₂

c. 35 SF₆ - 1 CO₂ - 64 CHClF₂

d. 98 SF₆ - 1 CO₂ - 1 CHClF₂

e. 24 SF₆ - 75 CO₂ - 1 CHClF₂.

There was the synergistic BDV effect within an area on the ternary diagram defined by a polygon c-d-f-c having at its corners the points defined by

c. 35 SF.sub. 6 - 1 CO₂ - 64 CHClF₂

d. 98 SF₆ - 1 CO₂ - 1 CHClF₂

f. 64 SF₆ - 35 CO₂ - 1 CHClF₂.

Carbon formation was suppressed for compositions lying in regions rich in SF₆ and CO₂ defined by a line b-c having at its extremities the points defined by

b. 1 SF₆ - 50 CO₂ - 49 CHClF₂

c. 35 SF₆ - 1 CO₂ - 64 CHClF₂.

EXAMPLE 23 System SF₆ -CO₂ -CBrF₃ (FIG. 6)

This system evidenced useful dielectric behavior within an area on a ternary diagram defined by a polygon a-b-c-d-e-a having at its corners the points defined by

a. 1 SF₆ - 65 CO₂ - 34 CBrF₃

b. 1 SF₆ - 15 CO₂ - 84 CBrF₃

c. 10 SF₆ - 1 CO₂ - 89 CBrF₃

d. 98 SF₆ - 1 CO₂ - 1 CBrF₃

e. 24 SF₆ - 75 CO₂ - 1 CBrF₃.

There was a synergistic BDV effect within an area on the ternary diagram defined by a polygon f-g-h-d-f having at its corners the points defined by

f. 50 SF₆ - 49 CO₂ - 1 CBrF₃

g. 50 SF₆ - 5 CO₂ - 45 CBrF₃

h. 54 SF₆ - 1 CO₂ - 45 CBrF₃

d. 98 SF₆ - 1 CO₂ - 1 CBrF₃.

Carbon formation was suppressed for compositions lying in regions rich in SF₆ and CO₂ defined by a line b-c having at its extremities the points defined by

b. 1 SF₆ - 14 CO₂ - 85 CBrF₃

c. 14 SF₆ - 1 CO₂ - 85 CBrF₃.

EXAMPLE 24 System SF₆ -CO₂ -CCl₂ FCClF₂

This system evidenced useful dielectric behavior within an area on a ternary diagram defined by a polygon having at its corners the points defined by

1 SF₆ - 74 CO₂ - 25 CCl₂ FCClF₂

25 sf₆ - 50 co₂ - 25 ccl₂ FCClF₂

38 sf₆ - 1 co₂ - 61 ccl₂ FCClF₂

98 sf₆ - 1 co₂ - 1 ccl₂ FCClF₂

25 sf₆ - 74 co₂ - 1 ccl₂ FCClF₂.

Carbon formation was suppressed for compositions lying in regions rich in SF₆ and CO₂ defined by two lines having at their extremities the points defined by

1. 1 SF₆ - 74 CO₂ - 25 CCl₂ FCClF₂

25 SF₆ - 50 CO₂ - 25 CCl₂ FCClF₂

2. 25 SF₆ - 50 CO₂ - 25 CCl₂ FCClF₂

38 SF₆ - 1 CO₂ - 61 CCl₂ FCClF₂.

EXAMPLE 25 System SF₆ -CO₂ -CClF₂ CClF₂ (FIG. 7)

This system evidenced useful dielectric behavior within an area on a ternary diagram defined by a polygon a-b-c-d-e-a having at its corners the points defined by

a. 1 SF₆ - 85 CO₂ - 14 CClF₂ CClF₂

b. 1 SF₆ - 45 CO₂ - 54 CClF₂ CClF₂

c. 45 SF₆ - 1 CO₂ - 54 CClF₂ CClF₂

d. 98 SF₆ - 1 CO₂ - 1 CClF₂ CClF₂

e. 24 SF₆ - 75 CO₂ - 1 CClF₂ CClF₂.

There was a synergistic BDV effect within an area on the ternary diagram defined by a polygon f-c-d-g-f having at its corners the points defined by

f. 11 SF₆ - 35 CO₂ - 54 CClF₂ CClF₂

c. 45 SF₆ - 1 CO₂ - 54 CClF₂ CClF₂

d. 98 SF₆ - 1 CO₂ - 1 CClF₂ CClF₂

g. 64 SF₆ - 35 CO₂ - 1 CClF₂ CClF₂.

Carbon formation was suppressed for compositions lying in regions rich in SF₆ and CO₂ defined by a line b-c having at its extremities the points defined by

b. 1 SF₆ - 45 CO₂ - 54 CClF₂ CClF₂

c. 45 SF₆ - 1 CO₂ - 54 CClF₂ CClF₂.

EXAMPLE 26 System SF₆ -CO₂ -CClF₂ CF₃ (FIG. 8)

This system evidenced useful dielectric behavior within an area on a ternary diagram defined by a polygon a-b-c-d-e-a having at its corners the points defined by

a. 1 SF₆ - 70 CO₂ - 29 CClF₂ CF₃

b. 1 SF₆ - 25 CO₂ - 74 CClF₂ CF₃

c. 25 SF₆ - 1 CO₂ - 74 CClF₂ CF₃

d. 98 SF₆ - 1 CO₂ - 1 CClF₂ CF₃

e. 24 SF₆ - 75 CO₂ - 1 CClF₂ CF₃.

There was a synergistic effect within an area on the ternary diagram defined by a polygon f-c-d-g-f having at its corners the points defined by

f. 35 SF₆ - 20 CO₂ - 45 CClF₂ CF₃

c. 25 SF₆ - 1 CO₂ - 74 CClF₂ CF₃

d. 98 SF₆ - 1 CO₂ - 1 CClF₂ CF₃

g. 79 SF₆ - 20 CO₂ - 1 CClF₂ CF₃.

Carbon formation was suppressed for compositions lying in regions rich in SF₆ and CO₂ defined by a line b-c having at its extremities the points defined by

b. 1 SF₆ - 25 CO₂ - 74 CClF₂ CF₃

c. 25 SF₆ - 1 CO₂ - 74 CClF₂ CF₃.

EXAMPLE 27

System SF₆ -CClF₃ -CHF₃ (FIG. 9)

The system evidenced useful dielectric behavior within an area on a ternary diagram defined by a polygon a-b-c-d-a having at its corners the points defined by

a. 10 SF₆ - 89 CClF₃ - 1 CHF₃

b. 25 SF₆ - 37.5 CClF₃ - 37.5 CHF₃

c. 10 SF₆ - 1 CClF₃ - 89 CHF₃

d. 98 SF₆ - 1 CClF₃ - 1 CHF₃.

Carbon formation was suppressed for compositions lying in regions rich in SF₆ defined by two lines, a-b and b-c, having at their extremities the points defined by

1. a. 10 SF₆ - 89 CClF₃ - 1 CHF₃

b. 25 SF₆ - 37.5 CClF₃ - 37.5 CHF₃

2. b. 25 SF₆ - 37.5 CClF₃ - 37.5 CHF₃

c. 10 SF₆ - 1 CClF₃ - 89 CHF₃.

This system is useful in gas filled transformers operating under winter conditions and in circuit breaker controls.

EXAMPLE 28 System SF₆ -CHF₃ -CHClF₂

This system evidenced useful dielectric behavior within an area on a ternary diagram defined by a polygon having at its corners the points defined by

20 SF₆ - 79 CHF₃ - 1 CHClF₂

44 SF₆ - 1 CHF₃ - 60 CHClF₂

98 SF₆ - 1 CHF₃ - 1 CHClF₂.

There was a synergistic BDV effect within an area on the ternary diagram defined by a polygon having at its corners the points defined by

45 SF₆ - 5 CHF₃ - 50 CHClF₂

49 SF₆ - 1 CHF₃ - 50 CHClF₂

98 SF₆ - 1 CHF₃ - 1 CHClF₂

94 SF₆ - 5 CHF₃ - 1 CHClF₂.

Carbon formation was suppressed for compositions having in regions rich in SF₆ defined by a line having at its extremities the points defined by

20 SF₆ - 79 CHF₃ - 1 CHClF₂

44 SF₆ - 1 CHF₃ - 60 CHClF₂.

EXAMPLE 29. System SF₆ -CCl₂ F₂ -CClF₂ CClF₂

This system evidenced useful dielectric behavior within an area on a ternary diagram defined by a polygon having at its corners the points defined by

10 SF₆ - 89 CCl₂ F₂ - 1 CClF₂ CClF₂

47 SF₆ - 1 CCl₂ F₂ - 52 CClF₂ CClF₂

98 SF₆ - 1 CCl₂ F₂ - 1 CClF₂ CClF₂.

Carbon formation was suppressed for compositions lying in regions rich in SF₆ defined by a line having at its extremities the points defined by

10 SF₆ - 89 CCl₂ F₂ - 1 CClF₂ CClF₂

47 SF₆ - 1 CCl₂ F₂ - 52 CClF₂ CClF₂.

EXAMPLE 30. System SF₆ -CCl₂ F₂ -CClF₂ CF₃

This system evidenced useful dielectric behavior within an area on a ternary diagram defined by a polygon having at its corners the points defined by

11 SF₆ - 88 CCl₂ F₂ - 1 CClF₂ CF₃

26 SF₆ - 1 CCl₂ F₂ - 73 CClF₂ CF₃

98 SF₆ - 1 CCl₂ F₂ - 1 CClF₂ CF₃.

There was a synergistic BDV effect within an area on the ternary diagram defined by a polygon having at its corners the points defined by

30 SF₆ - 55 CCl₂ F₂ - 15 CClF₂ CF₃

30 SF₆ - 1 CCl₂ F₂ - 69 CClF₂ CF₃

98 SF₆ - 1 CCl₂ F₂ - 1 CClF₂ CF₃

44 SF₆ - 55 CCl₂ F₂ - 1 CClF₂ CF₃.

Carbon formation was suppressed for compositions lying in regions rich in SF₆ defined by a line having at its extremities the points defined by

11 SF₆ - 88 CCl₂ F₂ - 1 CClF₂ CF₃

26 SF₆ - 1 CCl₂ F₂ - 73 CClF₂ CF₃.

EXAMPLE 31. System SF₆ -CClF₃ -CClF₂ CF₃

This system evidenced useful dielectric behavior within an area on a ternary diagram defined by a polygon having at its corners the points defined by

5 SF₆ - 85 CClF₃ - 10 CClF₂ CF₃

27 SF₆ - 1 CClF₃ - 72 -CClF₂ CF₃

98 SF₆ - 1 CClF₃ - 1 CClF₂ CF₃

14 SF₆ - 85 CClF₃ - 1 CClF₂ CF₃.

There was a synergistic BDV effect within an area on the ternary diagram defined by a polygon having at its corners the points defined by

30 SF₆ - 15 CClF₃ - 55 CClF₂ CF₃

30 SF₆ - 1 CClF₃ - 69 CClF₂ CF₃

98 SF₆ - 1 CClF₃ - 1 CClF₂ CF₃

84 SF₆ - 15 CClF₃ - 1 CClF₂ CF₃.

Carbon formation was suppressed for compositions lying in regions rich in SF₆ defined by a line having at its extremities the points defined by

1 SF₆ - 95 CClF₃ - 4 CClF₂ CF₃

27 SF₆ - 1 CClF₃ - 72 CClF₂ CF₃.

EXAMPLE 32. System SF₆ -CBrF₃ -CClF₂ CClF₂

This system evidenced useful dielectric behavior within an area on a ternary diagram defined by a polygon having at its corners the points defined by

14 SF₆ - 85 CBrF₃ - 1 CClF₂ CClF₂

45 SF₆ - 1 CBrF₃ - 54 CClF₂ CClF₂

98 SF₆ - 1 CBrF₃ - 1 CClF₂ CClF₂.

Carbon formation was suppressed for compositions lying in regions rich in SF₆ defined by a line having at its extremities the points defined by

14 SF₆ - 85 CBrF₃ - 1 CClF₂ CClF₂

45 SF₆ - 1 CBrF₃ - 54 CClF₂ CClF₂.

EXAMPLE 33. System CO₂ -CBrF₃ -CClF₂ CClF₂

This system evidenced useful dielectric behavior within an area on a ternary diagram defined by a polygon having at its corners the points defined by

15 CO₂ - 84 CBrF₃ - 1 CClF₂ CClF₂

45 CO₂ - 1 CBrF₃ - 54 CClF₂ CClF₂

85 CO₂ - 1 CBrF₃ - 14 CClF₂ CClF₂

50 CO₂ - 49 CBrF₃ - 1 CClF₂ CClF₂.

Carbon formation was suppressed for compositions lying in regions rich in CO₂ and defined by a line having at is extremities the points defined by

15 CO₂ - 84 CBrF₃ - 1 CClF₂ CClF₂

45 CO₂ - 1 CBrF₃ - 54 CClF₂ CClF₂.

EXAMPLE 34. System CO₂ -CF₃ CF₃ -c-C₄ F₈ (FIG. 10)

This system evidenced useful dielectric behavior within an area on a ternary diagram defined by a polygon a-b-c-d-e-f-g-a having at its corners the points defined by

a. 35 CO₂ - 64 CF₃ CF₃ - 1 c-C₄ F₈

b. 15 CO₂ - 70 CF₃ CF₃ - 15 c-C₄ F₈

c. 15 CO₂ - 50 CF₃ CF₃ - 35 c-C₄ F₈

d. 55 CO₂ - 1 CF₃ CF₃ - 44 c-C₄ F₈

e. 75 CO₂ - 1 CF₃ CF₃ - 24 c-C₄ F₈

f. 75 CO₂ - 15 CF₃ CF₃ - 10 c-C-₄ F₈

g. 50 CO₂ - 49 CF₃ CF₃ - 1 c-C₄ F₈.

There was a synergistic BDV effect with an area on the ternary diagram defined by a polygon h-c-d-j-i-h having at its corners the points defined by

h. 15 CO₂ - 65 CF₃ CF₃ - 20 c-C₄ F₈

c. 15 CO₂ - 50 CF₃ CF₃ - 35 c-C₄ F₈

d. 55 CO₂ - 1 CF₃ CF₃ - 44 c-C₄ F₈

j. 58 CO₂ - 1 CF₃ CF₃ - 41 c-C₄ F₈

i. 20 CO₂ - 60 CF₃ CF₃ - 20 c-C₄ F₈.

Carbon formation was suppressed for compositions lying in regions rich in CO₂ defined by three lines, a-b, b-c and c-d, having at their extremities the points defined by

1. a. 35 CO₂ - 64 CF₃ CF₃ - 1 c-C₄ F₈

b. 15 CO₂ - 70 CF₃ CF₃ - 15 c-C₄ F₈

2. b. 15 CO₂ - 70 CF₃ CF₃ - 15 c-C₄ F₈

c. 15 CO₂ - 50 CF₃ CF₃ - 35 c-C₄ F₈

3. c. 15 CO₂ - 50 CF₃ CF₃ - 35 c-C₄ F₈

d. 55 CO₂ - 1 CF₃ CF₃ - 44 c-C₄ F₈.

EXAMPLE 35. System CO₂ -CCl₂ F₂ -CHClF₂

This system evidenced useful dielectric behavior within an area on a ternary diagram defined by a polygon having at its corners the points defined by

50 CO₂ - 1 CCl₂ F₂ - 49 CHClF₂

25 CO₂ - 45 CCl₂ F₂ - 30 CHClF₂

20 CO₂ - 79 CCl₂ F₂ - 1 CHClF₂

75 CO₂ - 24 CCl₂ F₂ - 1 CHClF₂

64 CO₂ - 1 CCl₂ F₂ - 35 CHClF₂.

Carbon formation was suppressed for compositions lying in regions rich in CO₂ and defined by two lines having at their extremities the points defined by

1. 50 CO₂ - 1 CCl₂ F₂ - 49 CHClF₂

25 co₂ - 45 ccl₂ F₂ - 30 CHClF₂

2. 25 CO₂ - 45 CCl₂ F₂ - 30 CHClF₂

20 co₂ - 79 ccl₂ F₂ - 1 CHClF₂.

This system is an inexpensive gaseous dielectric mixture which, under a pressure of 1.5 atm, gives a dielectric strength approximately equal to SF₆ at 1 atm.

EXAMPLE 36. System 90 SF₆ -10 CO₂ -CCl₂ F₂ -CClF₂ CF₃

This quaternary system, in which SF₆ and CO₂ were held in a constant ratio of 90/10, evidenced useful dielectric behavior within an area on a ternary diagram of SF₆ -CO₂, CCl₂ F₂ and CClF₂ CF₃ defined by a polygon having at its corners the points defined by

10 SF₆ -CO₂ - 89 CCl₂ F₂ - 1 CClF₂ CF₃

30 SF₆ -CO₂ - 1 CCl₂ F₂ - 69 CClF₂ CF₃

98 SF₆ -CO₂ - 1 CCl₂ F₂ - 1 CClF₂ CF₃.

There was a synergistic BDV effect within an area on the ternary diagram defined by a polygon having at its corners the points defined by

40 SF₆ -CO₂ - 30 CCl₂ F₂ - 30 CClF₂ CF₃

30 SF₆ -CO₂ - 1 CCl₂ F₂ - 69 CClF₂ CF₃

98 SF₆ -CO₂ - 1 CCl₂ F₂ - 1 CClF₂ CF₃

69 SF₆ -CO₂ - 30 CCl₂ F₂ - 1 CClF₂ CF₃.

Carbon formation was suppressed for compositions lying in regions rich in SF₆ -CO₂ defined by a line having at its extremities the points defined by

10 SF₆ -CO₂ - 89 CCl₂ F₂ - 1 CClF₂ CF₃

30 SF₆ -CO₂ - 1 CCl₂ F₂ - 69 CClF₂ CF₃. 

What is claimed is:
 1. A process for suppressing carbon formation in a dielectric fluid during an electrical discharge from an electrical conductor which comprises contacting the electrical conductor during operation with a gaseous dielectric mixture consisting essentially of at least one halogenated alkane plus one member selected from the group consisting of CO₂, in an amount of at least 15 mole percent, and a combination of SF₆ and CO₂ which, when plotted on a ternary diagram in mole percent of SF₆ -CO₂ -halogenated alkane, lies in regions rich in SF₆ and CO₂ defined by a line having at its extremities the points defined by1 SF₆ - 15 CO₂ - 84 halogenated alkane 10 SF₆ - 1 CO₂ - 89 halogenated alkane,said halogenated alkane containing from 1 to 4 carbon atoms and at most one hydrogen atom, with the remaining hydrogen atoms replaced by at least one halogen selected from the group consisting of fluorine, chlorine and bromine, and having a vapor pressure of at least about 100 torr at 20° C.
 2. The process of claim 1 in which the halogenated alkane has a vapor pressure of at least about 400 Torr at 20° C.
 3. The process of claim 1 in which the halogenated alkane is totally gaseous at room temperature and has a boiling point of less than about 5° C.
 4. The process of claim 1 in which the halogenated alkane consists essentially of at least one compound selected from the group consisting of CHClF₂, CHF₃, CCl₃ F, CCl₂ F₂, CClF₃, CBrF₃, CClF₂ CClF₂, CClF₂ CF₃, CF₃ CF₃ and c-C₄ F₈.
 5. The process of claim 4 in which the gaseous dielectric mixture consists essentially of at least one halogenated alkane selected from the group consisting of CF₃ CF₃, CHClF₂, CCl₂ F₂ and CClF.sub. 2 CF₃ plus at least about 15 mole percent of CO₂.
 6. The process of claim 5 in which the gaseous dielectric mixture consists essentially of CCl₂ F₂ and about 15 to 65 mole percent of CO₂.
 7. The process of claim 5 in which the gaseous dielectric mixture consists essentially of CClF₂ CF₃ and about 25 to 70 mole percent of CO₂.
 8. The process of claim 4 in which the gaseous dielectric mixture consists essentially of at least one halogneated alkane selected from the group consisting of CCl₂ F₂, CHClF₂, CBrF₃, CCl₂ FCClF₂, CClF₂ CClF₂ and CClF₂ CF₃ plus both SF₆ and CO₂, the composition of the gaseous dielectric mixture, when plotted on a ternary diagram, lying in regions rich in SF₆ and CO₂ defined by a line having at its extremities the points defined by1 SF₆ - 15 CO₂ - 84 halogenated alkane 10 SF₆ - 1 CO₂ - 89 halogenated alkane.
 9. The process of claim 8 in which the gaseous dielectric mixture consists essentially of CC1₂ F₂, SF₆ and CO₂, the composition of the gaseous dielectric mixture being defined by the area enclosed by the polygon a-b-c-d-e-a in FIG. 4 of the attached drawings.
 10. The process of claim 8 in which the gaseous dielectric mixture consists essentially of CClF₂ CClF₂, SF₆ and CO₂, the composition of the gaseous dielectric mixture being defined by the area enclosed by the polygon a-b-c-d-e-a in FIG. 7 of the attached drawings.
 11. The process of claim 8 in which the gaseous dielectric mixture consists essentially of CClF₂ CF₃, SF₆ and CO₂, the composition of the gaseous dielectric mixture being defined by the area enclosed by the polygon a-b-c-d-e-a in FIG. 8 of the attached drawings.
 12. The process of claim 5 in which the gaseous dielectric mixture consists essentially of CF₃ CF₃ and about 35 to 50 mole percent of CO₂.
 13. A process for suppressing carbon formation in a dielectric fluid during an electrical discharge from an electrical conductor which comprises contacting the electrical conductor during operation with a gaseous dielectric mixture consisting essentially of at least one halogenated alkane selected from the group consisting of CCl₂ F₂, CHClF₂, CBrF₃, CClF₂ CClF₂ and CClF₂ CF₃ plus one member selected from the group consisting of CO₂, in an amount of at least 15 mole percent, and a combination of SF₆ and CO₂ which, when plotted on a ternary diagram in mole percent of SF₆ - CO₂ -halogenated alkane, lies in regions rich in SF₆ and CO₂ defined by a line having at its extremities the points defined by1 SF₆ - 15 CO₂ - 84 halogenated alkane 10 SF₆ - 1 CO₂ - 89 halogenated alkane,said gaseous dielectric mixture evidencing improved dielectric strength.
 14. A process for suppressing carbon formation in a dielectric fluid during an electrical discharge from an electrical conductor which comprises contacting the electrical conductor during operation with a gaseous dielectric mixture consisting of a mixture selected from the group consisting ofa. 40 to 80 mole percent of SF₆, balance CCl₂ F₂ ; b. 75 to 85 mole percent of SF₆, balance CClF₃ ; c. 60 to 85 mole percent of SF₆, balance CBrF₃ ; d. 40 to 50 mole percent of SF₆, balance CHClF₂ ; e. 25 to 90 mole percent of SF₆, balance CClF₂ CF₃ ; f. a composition within an area on a ternary diagram defined by a polygon having at its corners the points defined by45 SF₆ - 5 CHF₃ - 50 CHClF₂ 49 sf₆ - 1 chf₃ - 50 chcf₂ 98 sf₆ - 1 chf₃ - 1 chclF₂ 94 sf₆ - 5 chf₃ - 1 chclF₂ ; g. a composition within an area on a ternary diagram defined by a polygon having at its corners the points defined by30 SF₆ - 55 CCl₂ F₂ - 15 CClF₂ CF₃ 30 sf₆ - 1 ccl₂ F₂ - 69 CClF₂ CF₃ 98 sf₆ - 1 ccl₂ f₂ - 1 cclF₂ CF₃ 44 sf₆ - 55 ccl₂ F₂ - 1 CClF₂ CF₃ ; and h. a composition within an area on a ternary diagram defined by a polygon having at its corners the points defined by30SF₆ - 15 CClF₃ - 55 CClF₂ CF₃ 30 sf₆ - 1 cclF₃ - 69 CClF₂ CF₃ 98 sf₆ - 1 cclF₃ - 1 CClF₂ CF₃ 84 sf₆ - 15 cclF₃ - 1 CClF₂ CF₃.
 15. A carbon formation suppressant composition consisting essentially of at least one halogenated alkane plus both SF₆ and CO₂, said halogenated alkane containing from 1 to 4 carbon atoms and at most one hydrogen atom, with the remaining hydrogen atoms replaced by at least one halogen selected from the group consisting of fluorine, chlorine and bromine, and having a vapor pressure of at least about 100 Torr at 20° C, said composition, when plotted on a ternary diagram, lying in the regions rich in SF₆ and CO₂ defined by a line having at its extremities the points defined by1 SF₆ - 15 CO₂ - 84 halogenated alkane 10 SF₆ - 1 CO₂ - 89 halogenated alkane.
 16. The composition of claim 15 in which the vapor pressure of the halogenated alkane is at least about 400 Torr at 20° C.
 17. The composition of claim 15 in which the vapor pressure of the halogenated alkane is totally gaseous at room temperature and has a boiling point of less than about 5° C.
 18. The composition of claim 15 in which the halogenated alkane consists essentially of at least one compound selected from the group consisting of CHClF₂, CHF₃, CCl₃ F, CCl₂ F₂, CClF₃, CBrF₃, CClF₂ CClF₂, CClF₂ CF₃, CF₃ CF₃ and c-C₄ F₈.
 19. The composition of claim 18 consisting essentially of CCl₂ F₂, SF₆ and CO₂, the composition being defined by the area enclosed by the polygon a-b-c-d-e-a in FIG. 4 of the attached drawings.
 20. The composition of claim 18 consisting essentially of CHClF₂, SF₆ and CO₂, the composition being defined by the area enclosed by the polygon a-b-c-d-e-a in FIG. 5 of the attached drawings.
 21. The composition of claim 18 consisting essentially of CBrF₃, SF₆ and CO₂, the composition being defined by the area enclosed by the polygon a-b-c-d-e-a in FIG. 6 of the attached drawings.
 22. The composition of claim 18 consisting essentially of CClF₂ CClF₂, SF₆ and CO₂, the composition being defined by the area enclosed by the polygon a-b-c-d-e-a in FIG. 7 of the attached drawings.
 23. The composition of claim 18 consisting essentially of CClF₂ CF₃, SF₆ and CO₂, the composition being defined by the area enclosed by the polygon a-b-c-d-e-a in FIG. 8 of the attached drawings.
 24. The composition of claim 18 having improved dielectric strength, in which the halogenated alkane consists essentially of at least one compound selected from the group consisting of CCl₂ F₂, CHClF₂, CBrF₃, CClF₂ CClF₂ and CClF₂ CF₃.
 25. The composition of claim 24 consisting essentially of CCl₂ F₂, SF₆ and CO₂, the composition being defined by the area enclosed by the polygon f-g-d-h-f in FIG. 4 of the attached drawings.
 26. The composition of claim 24 consisting essentially of CHClF₂, SF₆ and CO₂, the composition being defined by the area enclosed by the polygon c-d-f-c in FIG. 5 of the attached drawings.
 27. The composition of claim 24 consisting essentially of CBrF₃, SF₆ and CO₂, the composition being defined by the area enclosed by the polygon f-g-h-c-f in FIG. 6 of the attached drawings.
 28. The composition of claim 24 consisting essentially of CClF₂ CClF₂, SF₆ and CO₂, the composition being defined by the area enclosed by the polygon f-c-d-g-f in FIG. 7 of the attached drawings.
 29. The composition of claim 24 consisting essentially of CClF₂ CF₃, SF₆ and CO₂, the composition being defined by the area enclosed by the polygon f-c-d-g-f in FIG. 8 of the attached drawings. 